diff --git a/src/logger.test.ts b/src/logger.test.ts
index 1c904cd..d718f80 100644
--- a/src/logger.test.ts
+++ b/src/logger.test.ts
@@ -411,3 +411,16 @@ function testNormalizeError(stringifiedError: string) {
     name: 'TypeError',
   });
 }
+
+test.skip('type checking', () => {
+  expect(true).toBe(true);
+
+  const lc = new OptionalLoggerImpl(consoleLogSink, 'debug', {foo: 'bar'});
+  const {debug} = lc;
+
+  // @ts-expect-error Should not allow extracted method.
+  debug?.('a');
+
+  // OK
+  debug?.call(lc, 'a');
+});
diff --git a/src/logger.ts b/src/logger.ts
index 54bab83..9bb84ab 100644
--- a/src/logger.ts
+++ b/src/logger.ts
@@ -11,11 +11,19 @@ export interface OptionalLogger<
   Info extends unknown[] = unknown[],
   Debug extends unknown[] = unknown[],
 > {
-  error?: ((...args: Error) => void) | undefined;
-  info?: ((...args: Info) => void) | undefined;
-  warn?: ((...args: Warn) => void) | undefined;
-  debug?: ((...args: Debug) => void) | undefined;
-  flush?: (() => Promise<void>) | undefined;
+  error?:
+    | ((this: OptionalLogger<Error, Warn, Info, Debug>, ...args: Error) => void)
+    | undefined;
+  info?:
+    | ((this: OptionalLogger<Error, Warn, Info, Debug>, ...args: Info) => void)
+    | undefined;
+  warn?:
+    | ((this: OptionalLogger<Error, Warn, Info, Debug>, ...args: Warn) => void)
+    | undefined;
+  debug?:
+    | ((this: OptionalLogger<Error, Warn, Info, Debug>, ...args: Debug) => void)
+    | undefined;
+  flush?(this: OptionalLogger<Error, Warn, Info, Debug>): Promise<void>;
 }
 
 /**
@@ -82,11 +90,21 @@ export class OptionalLoggerImpl<
   Info extends unknown[] = unknown[],
   Debug extends unknown[] = unknown[],
 > implements OptionalLogger<Error, Warn, Info, Debug> {
-  readonly debug?: ((...args: Debug) => void) | undefined = undefined;
-  readonly info?: ((...args: Info) => void) | undefined = undefined;
-  readonly warn?: ((...args: Warn) => void) | undefined = undefined;
-  readonly error?: ((...args: Error) => void) | undefined = undefined;
-  readonly flush: () => Promise<void>;
+  readonly debug?:
+    | ((this: OptionalLogger<Error, Warn, Info, Debug>, ...args: Debug) => void)
+    | undefined = undefined;
+  readonly info?:
+    | ((this: OptionalLogger<Error, Warn, Info, Debug>, ...args: Info) => void)
+    | undefined = undefined;
+  readonly warn?:
+    | ((this: OptionalLogger<Error, Warn, Info, Debug>, ...args: Warn) => void)
+    | undefined = undefined;
+  readonly error?:
+    | ((this: OptionalLogger<Error, Warn, Info, Debug>, ...args: Error) => void)
+    | undefined = undefined;
+  readonly flush: (
+    this: OptionalLogger<Error, Warn, Info, Debug>,
+  ) => Promise<void>;
 
   readonly #logSink: LogSink<Error, Warn, Info, Debug>;
   readonly #context: Context | undefined;